Many processes are known for controlling the permeability of subterranean hydrocarbon-bearing formations to improve the secondary recovery of hydrocarbons. Partially hydrolyzed polyacrylamide gels initiated by redox catalysts are used for permeability control in a number of these processes. U.S. Pat. No. 3,785,437 to Clampitt et al. injects an aqueous slug containing polymer and either a metal-containing oxidizing agent or a reducing agent into a formation. A second aqueous slug is sequentially injected into the formation, which contains only the second component of the redox catalyst system not contained in the first slug, i.e. either the oxidizing or reducing agent. The slugs mix in the formation to produce a cross-linked polymer gel. U.S. Pat. Nos. 3,795,276 to Eilers et al. and 4,040,484 to Hessert accelerate the cross-linking of the water-soluble polymer by adding an acid to decrease the pH of the gelation reaction.
U.S. Pat. No. Re. 29,716, to Clampitt et al. uses a polymer gel as a drilling fluid or well completion fluid. The gel is produced by adding the redox catalysts simultaneously to an aqueous mixture or sequentially adding the oxidizing agent and reducing agent in either order to the aqueous medium. The preferred method is to add the reducing agent to the mixture first.
Chyi-Gang Huang, M.S. Thesis, "A Study of the Gelation of Polyacrylamide Through the Participation of Chromium (III)", 1980, University of Kansas, investigates the subject of polyacrylamide gel preparation and is a good general reference on the subject.
In a non-gelling application, U.S. Pat. No. 3,039,520 to McKennon adds a polymer to a waterflood to increase the viscosity of the flood. A polymer such as PHPA is added above ground to the flooding water either in dried form or in the aqueous form directly as produced.
The above-cited references describe a number of processes using polymers and polymer gels for secondary oil recovery. However, none of these processes provide adequate control of the polymer gelation reaction. Processes which mix the gel-forming components in situ after sequential injection do not achieve sufficient mixing of the components, while processes which mix the gel-forming components above ground followed by injection require imprecise mechanisms to delay gelation until the mixture is in place. In either case, a weak gel results, which does not satifactorily control permeability, and hydrocarbon recovery from the waterflood is not significantly improved.
The problem not solved in the prior art is that of identifying the reaction parameters to which the gelation reaction is most sensitive and controlling those parameters. A process is needed to produce a gel sufficiently stable to control permeability in a subterranean hydrocarbon-bearing formation during a waterflood. More particularly a process is needed whereby the process parameters may be varied to produce gels of different viscosities and gelation times. A process is needed where the gel-forming components are uniformly mixed above ground and the mixture is injected into a subterranean formation to form a homogeneous gel in situ.